The present invention is related to a brake system of the type combining electric and friction brake control means in a manner wherein the electric brake means is supplemented by the friction brake means an amount dependent upon the inability of the electric brake means to continuously produce vehicle brake effort according to the brake command signal.
Conventional braking systems of the above-mentioned type may be placed in either of two classes, as represented by the prior art showings in FIGS. 1 and 2 of the drawings, wherein the brake command and the effective electric brake effort are compared by pneumatic means and by electric means, respectively, to determine the amount of friction braking required to satisfy the brake command. In FIG. 1, the brake control system comprises a pneumatic/electric transducer device 1 subject to a pneumatic brake command signal BV supplied from a brake valve or the like (not shown) and a vehicle load signal supplied from a load responsive device 2, which may be subject to fluid pressure from the vehicle air springs (not shown). The output of transducer 1 is an electrical signal representative of the brake command as modified in accordance with vehicle load conditions. An electrical brake control system 3 of the vehicle is operative responsive to the load modified brake command signal, with the degree of effectiveness of the electric brake being detected by torque converter 4. The output of the torque converter 4 thus represents a feedback signal indicative of the amount of electric brake effort actually produced. An amplifier 6 passes this feedback signal to an electric-pneumatic transducer 7, where it is converted back to a fluid pressure signal and supplied to a self-lapping type relay valve device 8 and specifically to the pressure chamber 12 of the relay valve.
While the above-mentioned electric brake feedback signal is being generated, the brake command signal BV is concurrently supplied via pipe 20 to a pressure chamber 13 of relay valve device 8. A diaphragm piston 12a subject to the fluid pressure in chamber 12 and a piston 13a subject to the fluid pressure in chamber 13 act opposingly to control the opening and closing of the relay valve supply and exhaust valve assembly 14 such that an output fluid brake pressure is derived proportional to the difference between the brake command and effective electric brake effort, such brake pressure representing the friction braking required to supplement the electric braking sufficiently to meet the vehicle brake command BV.
As a further feature of relay valve device 8, well known to those skilled in the art, there may be provided a chamber 15 to which fluid pressure from the vehicle air springs is supplied to effect movement of a diaphragm piston 16 an amount dependent upon the vehicle load. A roller 17 connected to move with piston 16 acts between a pair of pivotal levers 18, 18a to which diaphragm pistons 12a, 13a are connected, thereby varying the ratio between the brake command signal BV and electric brake feedback signal. In this way, the fluid brake pressure produced by relay valve device 8 to supplement the electric brake effort is adjusted according to the vehicle load condition.
A disadvantage attributed to a control system of this type is the poor response of the relay valve device to rapid changes in the effective intensity of the electric brake effort. Consequently, the blending of the electric and pneumatic brake effort tends to be erratic, due to excessive cycling of the pneumatic brake in attempting to supplement the electric brake so that passenger comfort is adversely affected.
In FIG. 2 of the drawings, there is shown another type of brake control system known in the art, which overcomes the above-mentioned disadvantage of the system of FIG. 1, but which has a different disadvantage. This FIG. 2 arrangement employs a conventional type relay valve device 9, which differs from relay valve device 8 of FIG. 1 in that there is no pneumatic comparison function provided in order to derive the friction brake requirement. The friction brake requirement is instead obtained via an electrical summing junction 5, where the brake command signal BV is connected via a branch line 19 and compared to the output of torque converter 4, which reflects the degree of electric brake effectiveness. Thus, comparison of the brake command and electric brake effectiveness produced in response to the brake command is accomplished electrically external of the relay valve instead of pneumatically within the relay valve device, as in FIG. 1. The error signal resulting from this electrical comparison represents an electrical signal to a corresponding fluid pressure signal that is supplied to the control chamber of relay valve device 9, which responds to produce fluid brake pressure accordingly. While this arrangement tends to eliminate the disadvantage found in the FIG. 1 arrangement, it is unacceptable in actual practice because a necessary condition of operation is that the electrical apparatus be absolutely free of malfunction. Otherwise, the friction brake requirement signal supplied to relay valve device 9 will be inaccurate or may fail to materialize at all.